When a photodiode is operating at wavelength beyond the bandgap of its absorber material, such as L band for germanium for example, the absorption efficiency tends to become very low. Undesirably, the resultant low responsivity limits the performance of GeSi photodiodes in the wavelength range from 1.55 μm to 1.6 μm. Further, for a normal incident photodiode to operate at very high data rate such as 25 Gb/s or higher, the absorption layer thereof needs to be very thin in order to reach a high 3 dB bandwidth. However, a thin absorber tends to degrade the responsivity of the photodiode and negatively impact the device performance.